In recent years, in the field of polyurethane foams useful for various applications, various researches and developments have been made in order to improve the characteristics suitable for the respective applications. For example, along with upgrading of automobile seats in order to improve the riding comfort of seat cushion, it is targeted to improve the rebound resilience, vibration characteristics, durability, etc. With respect to the vibration characteristics, the influence of car body vibration over a human body varies depending upon the vibration frequency. However, it is considered effective for improvement of the riding comfort to take damping of vibration particularly large in a frequency range (for example from 4 to 8 Hz or from 6 to 20 Hz) to which a human is sensitive. Further, it is considered that in order to improve these characteristics, a seat cushion is effective which employs a polyoxyalkylene polyol having a higher molecular weight than one heretofore produced.
On the other hand, as a seat cushion, a combination of a metal spring with a pad material made of a flexible polyurethane foam (hereinafter referred to simply as a flexible foam) has been used in many cases. However, in recent years, there has been a trend of employing an automobile seat so-called a deep foam type whereby a metal spring is abolished by imparting spring characteristics to the flexible foam itself to meet the demand for e.g. cost down, reduction of weight, etc. The deep foam type seat has become thick, since no metal spring is used in combination.
Further, the characteristics of the flexible foam have become a factor substantially influential over the sitting comfort and riding comfort of the seat. Namely, the static characteristics and dynamic characteristics as indices for the sitting comfort and riding comfort, have been regarded as important in the development of flexible foams. Among the static characteristics, it is particularly important to control the supported feeling at the initial stage of sitting and the bottom-hitting feeling at the final stage of sitting.
When a person actually sits on a seat provided with a pad material of flexible foam, the flexible foam will be compressed and deflected, and the position of e.g. the hip will sink to a certain height. As a method for measuring this static characteristic (the static sitting feeling), a test method may be employed wherein the deflection is measured in a load test in accordance with a performance test method for a pad material for an automobile seat according to JASO automobile standard B408-89 (1989), to obtain a load-deflection curve, or the deflection under a load of 500N (Newton Load) may be used which is obtained from a load-deflection curve obtained by the measurement employing a pressure plate in accordance with JIS E7104 (2002). This pressure plate is oval with a long diameter A of 300 mm, a short diameter B of 250 mm and a thickness C of at least 35 mm and is so-called Tekken Plate.
On the other hand, the polyoxyalkylene polyol to be used as a material for the flexible foam, is usually produced by ring opening polymerization of an alkylene oxide such as propylene oxide, using a sodium/potassium catalyst such as sodium hydroxide or potassium hydroxide and an initiator such as a polyhydric alcohol. By this production method, an unsaturated monool having an unsaturated bond (hereinafter referred to simply as a monool) will be formed as a by-product, and the amount of such a monool produced, will increase with an increase of the molecular weight of the polyoxyalkylene polyol (a decrease of the hydroxyl value).
In the case of a polyoxyalkylene polyol having a hydroxyl value of about 56 mgKOH/g which is commonly used as a material for resilient polyurethane foams, the amount of such a monool produced, is not so large as to bring about a problem. However, in the case of a polyoxyalkylene polyol having a high molecular weight and a low hydroxyl value, the amount of such a monool produced, will be problematic. Namely, in a case where a resilient polyurethane foam is produced by using a polyoxyalkylene polyol having a high monool content (a high total unsaturation value), there will be a problem such as a decrease in hardness or a worse in compression set of the produced foam, or a worse in curing property at the time of production of the foam. Further, even if it is attempted to produce a polyoxyalkylene polyol having a low hydroxyl value by using a sodium/potassium catalyst, the amount of the monool produced will be so large that such production will be practically difficult.
Under the circumstances, a method has been proposed wherein in order to improve the characteristics such as the riding comfort, durability, etc. for an automobile seat, a polyoxyalkylene polyol having a low monool content is used for the production of a resilient polyurethane foam (Patent Document 1).
However, it has been found that a resilient polyurethane foam represented by a deep foam type produced by using a polyoxyalkylene polyol having a low monool content, has an extremely high rebound resilience (rebound resilience of core portion: 71 to 74%), whereby the riding comfort is inadequate from the viewpoint of the occupant posture-stability performance or supporting performance during traveling. In order to solve such problems, an invention has been proposed to suppress the rebound resilience by a combined use of a polyoxyalkylene polyol having a low unsaturation value and a polyoxyalkylene polyol having a low molecular weight with a hydroxyl value of from 90 to 300 mgKOH/g (Patent Document 2), but the hysteresis loss has been relatively large at a level of from 25 to 33%, such being disadvantageous from the viewpoint of the durability.
Further, with a seat of the above-mentioned deep foam type structure, the load-deflection characteristics are substantially influenced by the flexible foam itself, and it will be a seat having a relatively small difference in deflection on pressure side of from 500 N to 900 N, when the deflection under load is measured when it is pressed by the above-mentioned pressure plate from above. A seat having a small difference in deflection presents a bottom-hitting feeling and thus showed a tendency that the evaluation of the riding comfort was poor. Therefore, with a seat of deep foam type, in order to increase the difference in deflection, the thickness of the foam was increased. As a technique to increase the difference in deflection without increasing the thickness of the foam, it was proposed to use a fluorinated surfactant having a perfluoroalkyl group structure (Patent Document 3). However, a problem has been pointed out such that the effects tend to be different depending upon the structure of the fluorinated surfactant.
On the other hand, in a method for producing a polyurethane foam by reacting a polyol component comprising a polyol, a catalyst, a blowing agent and other additives with a polyisocyanate component, it has been proposed to add a certain specific bifunctional secondary amine in order to produce a foam having a reduced inclination of the curve on pressure side at a deflection of 75% as an index for evaluating the bottom-hitting feeling, which has no bottom-hitting feeling, and has all of the flexibility, the sinking degree and the vibration characteristics well balanced. However, the supported feeling was insufficient, and the durability particularly wet set was inadequate (Patent Document 4).
Furthermore, a technique to increase the density of a surface skin layer of the foam by using a known defoaming agent, has been known (Patent Document 5). This technique is particularly applied to an integral skin foam and is substantially different from the present invention.
Further, a technique to increase the distribution of the size of cell by using an amino-modified silicone, has been known (Patent Document 6). This technique relates to a polyurethane foam for a polishing sheet, provides a foam having a specific gravity of 0.87, and is different from the present invention.
Patent Document 1:JP-A-7-330843Patent Document 2:JP-A-11-60676Patent Document 3:JP-A-11-322875Patent Document 4:JP-A-5-320304Patent Document 5:JP-A-6-87945Patent Document 6:JP-A-2004-75700